1. Technical Field of the Invention
The present invention relates to a control system for multiphase rotary electric machines for controlling output of a multiphase rotary electric machine by operating switching elements of the inverter.
2. Related Art
A control system is known in which output torque of a three-phase motor is controlled with the operation of switching elements of an inverter. The inverter in this control system has three arms for bringing each of the three phases of the three-phase motor into electrical conduction with either a positive or negative side of a power supply voltage. In the control performed by this system, a short circuit that has occurred in a switching element of any one of the arms of the inverter may cause current to pass through the arm, incurring the risk of fluctuating torque or increasing the heating value of the inverter.
Japanese Patent Application Laid-Open No. 6-319263 suggests a system having means for detecting current that flows through each of the arms of an inverter. This system enables determination on the occurrence of short circuit at a switching element in any one of the arms of the inverter. This patent document also suggests providing a breaker circuit between an inverter and a power source, so that a flow of current can be shut down between the inverter and the power source as to the phase the short circuit has occurred.
The conventional system mentioned above has a breaker circuit for the purpose of avoiding the problems of torque fluctuation and heating caused by the short circuit. Provision of such a breaker circuit, however, may unignorably increase the number of parts and the size of an inverter circuit.
Recently, a system for hybrid vehicles has been put into practical use. In this system, rotary shafts for a three-phase motor and a three-phase generator are linked to a rotary shaft of an internal combustion engine through a torque-splitting mechanism. This system is adapted to control the output torque of the three-phase motor by operating the inverter at the time of stopping the internal combustion engine, and to allow the vehicle to travel with this output torque. Also, when the internal combustion engine is in operation, this system is adapted to operate the inverter to control an amount of generated power of the three-phase generator.
In the above system as well, the occurrence of short circuit at any one of the arms of the inverter for the three-phase generator may cause the problems of torque fluctuations of the generator and increase in the heating value of the inverter. Further, when the torque of the generator and the motor is transmitted to the rotary shaft through the torque-splitting mechanism, the rotary shaft of the internal combustion engine is rotated even when the internal combustion engine is stopped, incurring the risk of vibrating the power transmission system. Also, when short circuit occurs at any one of the arms of the inverter for the three-phase motor, limp home processes may desirably be carried out by using the output torque of the internal combustion engine as a driving force. However, the occurrence of short circuit may introduce load torque to the three-phase motor to cause difficulty in appropriately transmitting the output torque of the internal combustion engine to the drive wheels.
The conventional control system mentioned above does not take measures for such problems involved in the system for hybrid vehicles.